Gelled hydrocarbons have been used in petroleum producing subterranean formations as fracturing fluids to improve the recovery of oil and natural gas. The fracturing fluids are hydraulically injected into a wellbore which penetrates the subterranean formation, and are propelled against the formation strata by high pressure, forcing the strata to crack and fracture.
The fracturing fluids carry proppant particles to the fracture site. These proppant particles remain in the fracture thereby "propping" the fracture open when the well is in production. The proppant material is commonly sand, sintered bauxite, polystyrene beads, and so forth, and is generally suspended in the gelled fluid due to the insolubility of the particles in the fluid.
Fracturing fluids may be thickened or gelled through the use of various chemical agents which act to increase viscosity or induce the gel formation. It is in fact widely accepted that the viscosity of liquid hydrocarbon fracturing fluids can be increased by a variety of thickening agents including fatty esters, orthophosphate esters, and aluminum complexed fatty acids.
Fatty ester gelling systems are for the most part, no longer utilized in oil field operations. The problem which was associated with these systems was the failure to generate adequate viscosity within the required time, i.e. the time required to pump the gel to the fracture site, therefore requiring aging of the gel prior to use.
A second class of gelling agents are the phosphate based compounds, including alkyl phophates coupled with aluminum and iron such as aluminum salts of orthophosphate esters. Aluminum crosslinked or iron crosslinked orthophosphate esters are probably the most commonly used gelling agents available today. They may be utilized in either a batch-mix or a continuous-mix process, and gel times can be quite fast. Most service companies do utilize a continuous mix process for the phosphate ester systems. However, there is an increasing demand in the oil industry for non-phosphate based gelling agents because phophates are known to be detrimental to the refining process.
Problems exist with the currently available systems as well. U.S. Pat. No. 5,614,010 issued Mar. 25, 1997 to Smith et al. describes problems associated with the use of aluminum compounds. Smith et al. state that aluminum will not satisfactorily perform the desired crosslinking function in the presence of more than about 1200 ppm of water, nor where the pH is outside a relatively narrow range. Smith et al. overcame this problem by utilizing ferric salts, rather than aluminum compounds for combination with orthophosphate ester.
The aluminum complexed fatty acids include aluminum octoates and aluminum stearates. A specific compound is the aluminum soap of 2-ethylhexoic acid, also referred to as aluminum octoate.
U.S. Pat. No. 5,417,287 describes iron crosslinked phosphate gels which exhibit fast gelling times.
U.S. Pat. No. 3,799,267 issued Mar. 26, 1974 to Ely et al. describes "a method whereby the viscosity of a liquid hydrocarbon, having an aluminum soap of an oil soluble aliphatic carboxylic acid solvated therein, is unexpectedly increased by dissolving benzoic acid in the hydrocarbon liquid." Ely et al. go on to state that "the addition of benzoic acid to a liquid hydrocarbon having an aluminum soap of an oil soluble aliphatic carboxylic acid solvated therein, has been found to further increase the viscosity of the liquid hydrocarbon by as much as 100 times the viscosity produced by an equivalent concentration of the aluminum soap solvated therein." See column 1, lines 39 to 50.
U.S. Pat. No. 4,981,608 issued Jan. 1, 1991 to Gunther describes the use of a gelling agent for a hydrophobic organic liquid comprising the solid reaction product of a poly-oxo-aluminum stearate and 2-ethyl hexanoic acid. Gunther states at column 2, lines 62 to 65 that "gelling occurs within a comparably very short time, such as one hour upon spraying or intermixing the gelling agent into the liquid to be gelled." Furthermore, many of the fatty acid systems are solids, while many end users prefer liquid based systems.
The present invention has overcome the aforementioned problems through the use of a hydrocarbon based gel which comprises at least one salt of a carboxylic acid having from about 6 to about 30 carbon atoms. Preferably, the salt is an aluminum tri-salt of 2-ethylhexanoic acid, and preferably at least one activator is utilized whereby the gelling can be controlled by the addition of the activator, and gelation can occur very quickly. The present invention utilizes a continuous mix gel process.